Neurons contact each other mostly by synaptic transmission at synapses. Synaptic transmission is mediated by calcium-triggered vesicle fusion with the plasma membrane, which releases transmitter molecules that act on postsynaptic transmitter receptors. My goal is to improve our understanding on the cellular and molecular mechanisms underlying synaptic vesicle exocytosis, which are the building block for synaptic transmission and thus the signaling process in the neuronal network. My progress in the last year is mostly in the study of the significance of fusion modes. In particular, we studied how the compound fusion contributes to the generation of posttetanic potentiation. Posttetanic potentiation, is a form of synaptic plasticity widely observed at synapses that may be involved in information processing of neuronal circuits. It can be induced by trains of action potential stimulation at 50-200 Hz for 1-30 seconds. Protein kinase C (PKC) has been implicated in playing a role in post-tetanic potentiation. However, whether PKC is responsible for generation of all PTP or only partially is unclear. We have recently showed that post-tetanic potentiaion (PTP) of the evoked, AMPA-receptor mediated EPSC was accompanied by calcium-dependent potentiation of the mEPSC amplitude at the calyx of Held synapse. The mEPSC amplitude (quantal size) potentiation is caused by compound vesicel fusion. It was smaller than the PTP in the first minute after tetanic stimulation, but became similar to the PTP 1 minute later, suggesting that in addition to the increased vesicle number, an increase in the quantal size contributes to the generation of PTP. However, two issues remained unresolved. First, it is unclear whether the calcium-dependent quantal size increase is also caused by activation of protein kinase C in a similar way as the calcium-dependent increase of the quantal content. Second, recent studies suggest that spontaneous and evoked release originate from different vesicle pools at hippocampal synapses. These studies suggested caution in using quantal analysis, a traditional analysis of evoked release based on the assumption that the spontaneous quantal event is the same as the quantal event during evoked release (Zucker, 2005;Rothwell, 2010). Consequently, additional evidence is needed to strengthen the suggestion that quantal size increase, detected as an increase of the mEPSC amplitude, contributes to the generation of PTP. We have addressed the above two issues by recording both the mEPSCs and EPSCs at a large central synapse, the calyx of Held synapse. We found that PTP of the EPSC is caused by two different cellular mechanisms, a PKC-dependent increase of the quantal content and a PKC-independent increase of the quantal size. Furthermore, we found that mEPSCs and EPSCs were subjected to similar up- and down-regulation, which verifies the basic assumption of quantal analysis--the same mechanism controls the quantal size of spontaneous and evoked release. This verification supports the use of quantal analysis at central synapses. However, unlike the traditional quantal analysis that attributes the quantal size change to a postsynaptic mechanism, this piece of work, together with one of our previous studies, suggests that the quantal size increase is caused by a presynaptic mechanism, the compound fusion among vesicles that forms large compound vesicles. In summary, our recent progress has firmly establised a new principle at synapses. This new principle is that Vesicles can fusion with each other to form giant vesicels during high frequency stimulation. The resulting giant vesicles generate large mEPSCs and thus potentiates synaptic transmission. Such a novel mechanism underlies a large part of PTP via a PKC-independent pathway. Teh remaining part of the PTP is caused by a PKC-dependent, but compound fusion-independent pathway.